1. Field of the Disclosure
The present disclosure pertains to logging while drilling apparatus and more particularly to acoustic logging while drilling apparatus and generating and using guided waves to look ahead of the drillbit.
2. Summary of the Related Art
To obtain hydrocarbons such as oil and gas, wells or wellbores are drilled into the ground through hydrocarbon-bearing subsurface formations. Currently, much current drilling activity involves not only vertical wells but also drilling horizontal wells. In drilling, information from the well itself must be obtained. While seismic data has provided information as to the area to drill and approximate depth of a pay zone, the seismic information can be not totally reliable at great depths. To support the data, information is obtained while drilling through logging while drilling or measuring while drilling (MWD) devices. Logging-while-drilling, or measuring-while-drilling are procedures that have been in use for many years. This procedure is preferred by drillers because it can be accomplished without having to stop drilling to log a hole. This is primarily due to the fact that logging an unfinished hole, prior to setting casing if necessary, can lead to washouts, damaging the drilling work that has already been done. This can stall the completion of the well and delay production. Further, this information can be useful while the well is being drilled to make direction changes immediately. Measurements, however, are taken long after the actual drilling of the well.
An important part of drilling operations is the trying to control the direction of drilling in a desired direction. This requires the ability to “see ahead” of the drillbit. Resistivity methods have been widely used for the purpose but these usually do not have the capability of seeing very far ahead of the drillbit. See, for example, U.S. RE 35389 to Wu et al., U.S. patent application Ser. No. 11/072,027 of Tabarovsky et al., U.S. patent application Ser. No. 11/183,139 of Folberth et al., U.S. patent application Ser. No. 11/489,875 of Wang et al. Formation pressure measurements have been used as in U.S. Pat. No. 7,063,174 to Chemali et al. The present disclosure deals with an acoustic method of looking ahead of the drillbit. For other examples using acoustic measurements, see U.S. Pat. No. 6,166,994 to Jeffryes, U.S. Pat. No. 5,678,643 to Robbins et al., U.S. Pat. No. 6,084,826 to Leggett et al., WO01/27657 of Kroode et al., and US 2004/0059512 of Thomann et al.